1. Field of the Invention
The present invention relates to a color image forming apparatus of an electrophotographic process such as a color printer or a color copying apparatus, and a control method therefor.
2. Related Background Art
For the color image forming apparatus employing an electrophotographic process or an ink jet process such as a color printer or a color copying apparatus, there is recently required a higher image quality for the output image. In particular, a density gradation and stability thereof influence significantly on the human judgment whether an image is good or not.
However, in the color image forming apparatus, an obtained image shows a variation in the density, in case a variation is caused in various units of the apparatus by an environmental change or by a prolonged use. Particularly in a color image forming apparatus of an electrophotographic process, even a slight environmental change may result a density variation leading to an aberration in the color balance, so that there is required means for always maintaining a constant density. There is therefore provided such construction as to form a density detecting toner image (hereinafter called patch) with toner of each color on an intermediate transfer member or a photosensitive member, to detect the density of such unfixed toner patch with an unfixed toner density detecting sensor (hereinafter called density sensor) and to execute a density control by feedback of a result of such detection to process conditions such as an exposure amount, a developing bias, etc., thereby obtaining stable images.
However, the density control utilizing such density sensor is based on detecting a patch formed on the intermediate transfer member or the photosensitive drum, and cannot control an aberration in the color balance resulting from variations in a transfer and a fixation on a transfer material to be executed thereafter. Such variations cannot be coped with the aforementioned density control utilizing the density sensor.
Consequently, there can be conceived a color image forming apparatus equipped with a sensor (hereinafter called color sensor) for detecting a density or color of a patch formed on the transfer material.
Such color sensor is constituted of three or more light sources with different light emission spectra such as light-emitting elements of red (R), green (G) and blue (B) or a light source such as a light-emitting element emitting a white (W) light, and light-receiving elements bearing three or more filters of different spectral transmittances such as of red (R), green (G) and blue (B). In this manner there can be obtained three or more different outputs such as RGB outputs.
However, a control with such color sensor requires a patch formation on the transfer material, thus necessitating consumption of a transfer material and toners. Consequently a frequency of such control cannot be made very high. Such color sensor only is unable to achieve an effective density control while minimizing the frequency of control with such color sensor.